Medical imaging methods for imaging the heart and the blood vessels in the vicinity of the heart in particular generally have the problem that the body region to be recorded is subjected to a continuous cyclical motion as a result of the heart beat. These circumstances are problematic, particularly in the case of X-ray tomographic recording methods in which the image data record to be generated is calculated by back-projection of a multiplicity of projection images recorded from different projection angles. The image data record generally represents a slice- or volume image of the recording region. The back-projection only works without faults if the projection images on which it is based image a body region during identical motion phases of the heart.
This therefore necessitates synchronizing the recording of the projection images with the cardiac motion of the patient in the case of X-ray tomographic recordings of the heart and/or the blood vessels in the vicinity of the heart such that the imaging is performed in each case during the same motion phase, preferably in the rest phase of the cardiac cycle. Here, corresponding “triggering” is brought about on the basis of the EKG (electrocardiogram) signal from the patient. The EKG-supported control of the imaging is also referred to as “EKG gating”. Here, the EKG-supported modulation of the tube current is referred to as “EKG pulsing”.
Since it is only the projection images recorded during the rest phases of the cardiac motion that are included in the reconstruction of the image data record, X-ray radiation need only be applied to the patient during the time windows required for registering the projection images. In order to reduce the applied X-ray dose, attempts are therefore made to reduce the radiation as far as possible outside of these time windows. Thus, the imaging methods with EKG pulsing therefore estimate the temporally next time window, i.e. the time window following immediately in the future, on the basis of a certain number of preceding cardiac cycles, and a recording pulse corresponding to the time window calculated in advance is generated in order to modulate the tube current during the imaging.
In order to ensure that the image information required for the image reconstruction is also registered in the case of an erroneous estimate of the time window, the X-ray radiation is generally not switched off completely outside of the predicted time windows but reduced to a certain fraction, e.g. 25%. This allows the set of projection images to be completed by using image information from outside of the time window for reconstructing the image data record, although this is generally connected with a significantly reduced image quality of the reconstructed image data record, in particular with increased image noise.
The time window used for generating the recording pulse is usually determined on the basis of the estimated cycle duration of the next cardiac cycle. In the process, the cycle duration is estimated from a predetermined number of preceding cardiac cycles, with the cycle duration corresponding to the time interval between two successive R-waves (or R-spikes). This time interval is also referred to as the “RR-interval”.
The mean value or the median value of the RR intervals determined in this fashion is used as subsequent time window for the recording pulse in the case of conventional imaging methods. Irregularities in the cardiac rhythm, i.e. a change in the cardiac frequency on the timescale, lead to an erroneous calculation of the predicted time windows in this method and thus lead to an impairment in the image quality of the image data record that can be obtained.
Therefore, in order to obtain an improved image quality in the reconstructed image data record, even when there is an irregularity present in the cardiac rhythm, DE 10 2005 036 963 B3 proposes to also take into account a dispersion parameter in the calculation of the time window for the recording pulse, which dispersion parameter characterizes the variation in the cycle duration. By way of example, such a dispersion parameter can be the standard deviation of the cycle duration in respect of a trend of the analyzed cardiac cycles. Here, the time window used to generate the recording pulse, that is to say the length of the time interval at full dose, is increased as the variation in the cycle duration increases, as a result of which the motion phase within the cardiac cycle preselected by the user is hit with increased reliability.
Hence, the projection images required for reconstructing the image data record at a preselected motion phase of the heart can also be obtained from the time window range at a high tube current even if there are variations in the cycle duration, and so a reconstruction of the image data record with an improved image quality is possible. However, the improved image quality in the case of an irregular cardiac rhythm is obtained at the expense of an increased X-ray dose compared to the conventional dose modulation methods because of the increase in the time window during which there is an irradiation of the patient at a high tube current.